High-pressure short-arc discharge lamps for projection purposes, in particular for video projection, are known for example from the company OSRAM under the name P-VIP®. In the case of such high-pressure discharge lamps, the electrodes are exposed to high thermal stresses and high currents, for example 4 amperes or more. Undesired electrode burn-back therefore occurs, electrode material on the electrode tip being evaporated, as well as likewise undesired migration of the electrode tips and consequently also of the discharge arc burning between the electrode tips. This migration of the electrode tip is represented very schematically in FIGS. 8a and 8b. FIG. 8a shows a highly simplified shape of an electrode 101 having an electrode rod 102 and an electrode head 103. The electrode head 103 comprises a circular-cylindrical main section 104, and a hemispherical end section 105 as the “electrode tip”. The relatively bulky main section 104 is used primarily for thermal radiation and therefore preferably has a relatively large surface area in comparison with the much smaller end section 105 (also referred to below as an electrode tip for the sake of simplicity), which is primarily used for maximally optimal positioning of the discharge arc and stable burning behavior thereof without arc jumps. In the course of operation of AC lamps, however, it is found that the end section 105, which melts at least partially during the lamp operation, can migrate on the end surface 106 of the main section 104. This effect can be commensurately more significant when the difference between the diameter of the end surface 106 and the diameter of the hemispherical end section 105 is greater. FIG. 8b shows by way of example the way in which the hemispherical end section 105 has migrated as far as the upper edge of the end surface 106 of the main section 104. This migration of the electrode tip generally leads at most to a minor change in the electrode spacing, i.e. the spacing of the electrode tips lying opposite one another in the discharge vessel. Measurement of the electrode spacing-dependent lamp voltage therefore likewise shows scarcely any change. Nevertheless, this effect of the electrode tip migration can lead to a reduction of, for example, 30% in the projector light. The reason for this is as follows. Since discharge lamps for projection applications are operated in an optical reflector, the migration of the electrode tips, and consequently of the discharge arc, leads to a significant reduction in the optical system efficiency since the luminous discharge arc in this case migrates from the primary focus of the elliptical reflector. In applications which require coupling of the light into an optical aperture arranged at the secondary focus of the reflector, for example the aperture of a DLP™ (DLP=Digital light processing), LCD or LCOS chip, or an optical integrator or light guide, the input coupling efficiency thereby furthermore decreases. This disadvantageous effect becomes more pronounced with an increasing reflector eccentricity, or decreasing input aperture of the downstream optical projector system.
In practice, electrodes having a hemispherical or frustoconical electrode head are predominantly used in the case of video projection lamps. Electrodes of the former type (see for example US 2004/0155588 A1) have a comparatively large mass in the front region of the electrode head, so that the electrode tip becomes less hot and less electric material consequently evaporates. They therefore generally have advantages in relation to the electrode burn-back behavior. However, they offer a relatively large surface for the electrode tip migration, so that the advantage in the burn-back behavior is generally outweighed by the disadvantage of the increased electrode tip migration.
Electrodes having a frustoconical electrode head, on the other hand, ensure stabilization of the electrode tip position by their tapering shape. Owing to the lower mass in the vicinity of the electrode tips, however, they generally exhibit significantly faster electrode burn-back. Accordingly, attempts have been made in the past to find the best possible compromise between electrode burn-back and electrode tip migration for a specific electrode by varying the cone angle.